1. Field of the Invention
The present invention relates to a three-dimensional image generating technique which obtains data necessary for generating a three-dimensional image of a real existing object by means of CG (computer graphics), and more specifically, a technique which adds a bump map to a three-dimensional shape model shown by a polygon mesh.
2. Description of Related Art
Conventionally, to reproduce a real existing object by means of CG, a method in which an image of the real existing object taken by a camera is pasted on the portions of a three-dimensional shape model has been used. In such a method, in order to cope with CG of various observing points, a plurality of images taken from a plurality of directions are prepared, and for reproduction, appropriate images are selected and pasted (refer to Japanese Patent Application Laid-Open No. 2000-348213).
However, by this method, images of an object to be reproduced by means of CG, illuminated under illumination conditions different from those when the object is imaged, cannot be obtained.
Furthermore, images taken by a camera originally include shades and highlights, so that an object pasted with the taken images becomes unnatural when it is provided with further shades and highlights.
Therefore, a method is employed in which a change in reflectance due to a light source direction or an observing direction is expressed as a reflection function, a numerical expression model is introduced for the reflection function, and a surface reflectance properties is indicated by reflection model constants (refer to “Object Shape and Reflectance Modeling from Observation:” SIGGRAPH Computer Graphics Proceedings, Annual Conference Series, pp379-387, 1997).
This method realizes reproduction of natural shades and highlights in optional illumination conditions and observing directions by use of comparatively slight surface reflectance properties information by holding reflection characteristics of colored portions provided on the surface of a three-dimensional shape model as reflection model constants, and when reproducing by substituting the constants into a reflection model function.
However, the abovementioned reflection model expresses light reflection and diffusion reflecting the surface structure at the micro level. Therefore, it cannot reflect fine shades due to unevenness that can be viewed, such as a stone surface, loosely woven cloth, and leathery crepe, etc. Of course, such unevenness can be expressed by a polygon mesh, however, in this case, the number of vertices and the number of polygons become enormous and result in a high operation processing burden, and this is not realistic.
On the other hand, in CG drawing, for drawing the abovementioned fine shades, a method called bump mapping is employed.
Generally, bump mapping is not concerned with shape drawing, but is used for shading by influencing normal directions at fine portions.